The present invention is directed to forming patterns in photo-imageable layers, such as for example photoresist layers. Photoresists, as well as all photo-imageable materials, are generally classified into two groups: positive-type materials and negative-type materials. Positive-type materials become soluble in a developer solution (e.g., alkaline-aqueous solution) when exposed to light radiation, usually ultraviolet. Negative-type materials become insoluble in a developer solution (e.g., an organic solvent) when exposed to light radiation. In either case, the light generally must be within a given range of wavelengths, the range being a function of the chemical and optical properties of the particular photo-imageable material. Light within the wavelength range is often referred to as "actinic" radiation. As used herein, actinic radiation for a given material is any electromagnetic radiation having a wavelength capable of activating that material.
A typical negative photoresist comprises a synthetic polymer resin (e.g., cyclized polyiosoprene rubber), a photo-initiator chemical (e.g., azide compounds), and a solvent base. The solvent base dissolves the resin and initiator chemical, which are usually solid or near solid. This enables the photoresist to readily coat a desired substrate (e.g., semiconductor wafer, ceramic substrate) to form a photoresist layer thereon. After coating, the solvent is evaporated from the substrate by exposure to moderate heat ("soft bake"), leaving the resin and photo-initiator as a hard layer.
The resin of a negative-type photoresist generally comprises polymer chains, each chain having one or more unsaturated carbon bonds. With sufficient energy, unsaturated bonds from two such chains may be bonded to form a saturated carbon bond. This cross-inks the chains and, in combination with other cross-links to other chains, renders the photoresist substantially insoluble in a selected organic solvent. The energy is generally provided by way of the photo-initiator chemical. Upon exposure to actinic radiation, the molecules of the photo-initiator absorb energy from the radiation and interact with chains to form cross links. Some resins are capable of cross-linking without the aid of a photo-initiator and are directly responsive to the actinic radiation.
A typical positive photoresist comprises a resin, a photo-sensitizer chemical, and a solvent base. As before, the solvent base dissolves the resin and sensitizer chemical, and is evaporated after the wafer has been coated to form a layer thereon. The resin's polymer chains are normally insoluble in alkaline-aqueous solutions. Upon exposure to actinic radiation, the molecules of the photo-sensitizer decompose into acidic products which, in the presence of an alkaline-aqueous solution, promote the dissolving of the polymer chains. The exposed portions of the positive-type photoresist then become soluble in an alkaline aqueous solution.
In use, a typical photoresist (either positive- or negative-type) is coated on the wafer to form a photoresist layer, soft-baked to remove the solvent base from the layer, and then exposed to actinic radiation through a mask. The mask transfers its pattern to the photoresist layer. The photoresist layer is then developed by exposure to an appropriate developer solution. For a negative photoresist, the unexposed portions of photoresist dissolve in the developer solution. For a positive photoresist, the exposed portions dissolve. A pattern is then left on the wafer for further processing. Once the remaining photoresist layer is no longer needed, it may be removed by exposure to an organic solvent called a stripper.
In addition to positive- and negative-type photoresists, there are image-reversal (or reversing) photoresists (IRP's), which are positive-type photoresists which can be post-treated to reverse the initially exposed images therein. IRP resists are also called dual tone resists, and are classified as positive resists because their chemical structures are closest to those of positive photoresists. In an unexposed state, IRP materials are insoluble in alkaline aqueous solutions (the developers). After exposure to actinic radiation through a mask, the exposed portions become soluble in an alkaline aqueous solution. The unexposed portions remain insoluble. The IRP material may then be developed as a positive resist. In the alternative, the resist may be heated to a temperature of around 100.degree. C.-160.degree. C. to render the exposed portions insoluble. By then exposing the previously unexposed portions to the actinic radiation, the previously unexposed portions are rendered soluble in the alkaline aqueous solution. Accordingly, the initial mask image is reversed, and may be developed to form a negative image of the mask. The second exposure to actinic radiation may expose the entire layer ("blank flood" exposure) rather than just the previously unexposed portions.
It is known that forming pattern features with aspect ratios of greater than 2 in thin photoresist layers (&lt;2 .mu.m in thickness) is difficult due to optical diffraction effects. (The aspect ratio is the thickness of the photoresist layer, i.e., height, divided by the narrowest width of the feature.) It is known that diffraction effects become less of a problem as the width of the feature increases, which implies that it should be easier to form high aspect ratio structures in thicker photoresist layers. Although the inventors have generally found this to be true, they have found in some instances that a small residue of photoresist material remains at the bottom of a high-aspect ratio feature formed in a thick photoresist layer. These residues are approximately 0.2 .mu.m thick, and have been observed most often when the thickness of the photoresist layer exceeds 60 .mu.m and when the feature width at the top of the photoresist is less than 10 .mu.m. Such residues generally interfere with subsequent processing, such as when the feature is filled with conductive material by electroplating, and with the function intended for the feature.